The currently produced idling control arrangement in accordance with the attached FIG. 1 has a throttle flap 10 which can be adjusted by means of an adjusting cable 11 or a servomotor 12. The setting angle ALPHA of the throttle flap 10 is determined from the voltage which is picked off at potentiometer terminals 13 from a potentiometer connected to the pivot axis of the throttle flap 10.
The servomotor 12 has an actuating housing 14 in which an actuating shaft 15 is arranged in such a way that it can be pushed back and forth. A part which can likewise be pushed back and forth, namely an idling-contact pin 16, is arranged in the actuating shaft 15 itself. In the position illustrated in FIG. 1, the free end of this contact pin 16 presses on an adjusting screw 17, which is threadably engaged in an actuating flange 18 connected to the pivot pin of the throttle flap 10. The actuating shaft 15 is displaced via a schematically represented electric drive 19 which either retracts or extends the actuating shaft 15 depending on the polarity of the voltage supplied to the drive. With respect to the explanation below, it is assumed that, deviating from the illustration according to FIG. 1, the actuating shaft 15 has been retracted to such an extent that the idling-contact pin 16 does not press on the adjusting screw 17. In this position, the idling-contact pin 16 is pushed out of the actuating shaft 15 as far as a stop (not shown) by a spring 20. It then does not press on an idling contact 21, which is therefore open. If the drive 19 is then supplied with voltage in such a way that it displaces the actuating shaft 15 to such an extent that the idling-contact pin 16 comes to rest against the adjusting screw 17, no rotation of the throttle flap 10 initially takes place as the actuating shaft 15 is pushed further out. Instead, the idling-contact pin 16 is first pushed into the actuating shaft 15 until it closes the idling contact 21 and cannot be closed any further. Only then is the throttle flap 10 rotated as the actuating shaft 15 is extended further. The illustration according to FIG. 1 corresponds to a position in which the idling contact 21 is closed.
If the servomotor 12 is in a certain position and the idling contact 21 is closed but the accelerator pedal is then pressed, that is, the adjusting cable 11 ensures a further opening of the throttle flap 10, the adjusting screw 17 lifts off from the idling-contact pin 16. As a result, the idling-contact pin 16 is pressed out of the actuating shaft 15 as far as its forward stop by the spring 20 and, as a result, the idling contact 21 opens. If this opening does not occur due to a fault or occurs but cannot be detected due to a short-circuit in the idling-contact drive circuit, the idling control arrangement remains active although idling conditions no longer exist. In order to detect this fault, the throttle flap angle ALPHA is monitored as to whether it reaches an angular range which characterizes full-load operation. If this is the case but the idling contact 21 is still closed, this indicates a fault in the idling-contact monitoring arrangement. As soon as this fault is detected, the actuating shaft 15 is retracted completely and then extended for a predetermined period so that it reaches a position in which, with the adjusting screw 17 resting against the idling-contact pin 16, the throttle flap assumes an angle ALPHA.sub.-- EMERGENCY, which is sufficiently large to ensure that the internal combustion engine driven with the aid of the arrangement cannot stall. Idling control is simultaneously suppressed.
This method is relatively unreliable with respect to the detection of the short-circuit fault mentioned and it is completely incapable of detecting the converse fault, namely when the idling contact remains open continuously, or of detecting faults associated with the functioning of the servomotor.